Nobody ridiculed the TSA for matching up an ostensibly
singular quantifier (everyone)
with an ostensibly plural personal pronoun (their). That sort of thing is old
hat and hardly worth remarking upon (though Lord knows that doesn't
stop us from remarking upon it, e.g., here,
here,
here,
here,
here,
here,
here,
here...).
The humor, such as it is, instead hinges on a supposed ambiguity in determining the
domain of the
quantifier everyone.

For the bloggers' joke to work, we have to toss out our Gricean maxims
and apply an extremely uncooperative
reading to the TSA's injunction. In a more explicit
metalanguage, we could render the obviously intended proposition as:

For every person xsuch thatx is traveling by air in the
U.S. with gel-filled
brasy, we
encourage x to pack
y in x's checked baggage.

The willfully obtuse interpretation given by the bloggers would go
something like this:

For every personxsuch that xis traveling by air in the
U.S., we encouragex
to pack gel-filled brasy in x's
checked baggage.

How do we know how to restrict the domain of a quantifier like everyone?
We rely on a variety of contextual clues. In this case, since the
sentence appears in
a list of instructions for passengers, we know that everyone must at least be limited
to the set of people traveling via U.S. airports, as even the
disingenuous reading recognizes. Elsewhere in the TSA text, those
domestic
air travelers are given instructions through a series of law-like
statements addressed
to a generic you, e.g.,"You
are permitted to bring solid cosmetics and personal hygiene items as
such lipstick, lip balm and similar solids," or this bizarre bit of advice: "We also ask that you follow the guidelines above and try not to over-think these guidelines." (I tried not to over-think but failed. Sorry, TSA — over-thinking is de rigueur here at Language Log Plaza.)

In the ridiculed sentence, the TSA text shifts to a different type
of indirect
speech act. Instead of second-person address directed to a generic you, an "encouragement" is given to
a generic everyone. But only
a subset of passengers was meant to be selected by the use of everyone: those traveling with
gel-filled bras, who must then decide where to pack them. The writer of
the text could have made this shift of
address more explicit by specifying the object of encouragement as
"everyone
traveling with gel-filled bras," but then that pesky coreference
of everyone with their might have gotten in the way:

The possible confusion between the two different coreferents of them and their would have been spared if the
writer had chosen a singular pronoun her
to match the antecedent everyone:
"pack them in her checked
baggage." But then the sudden change of addressee from all passengers
to only female ones might have sounded a bit unusual. Of course, the
non-gender-specific alternative his
or her is no help here in avoiding the gender-bending ridicule
of Schneier et al. In any case, I'm pretty sure the TSA wants to insulate itself from the tricky question of whether only women might be packing gel-filled bras. (This might explain why singular their was used in the first place.) It looks like the whole sentence would need to be recast in order to bypass all of these pitfalls.

The wisecrack at the TSA's expense actually strikes at the heart of a long-standing conundrum in the philosophy of language: how do we determine the intended domain of everyone and similar quantifiers? In the
Anglo-American philosophical tradition, the question has been posed as a challenge to
Bertrand Russell's theory of
descriptions. Russell's theory is not well-equipped to deal with incomplete
descriptions where quantifier domains must be inferred from pragmatic
context. For more on the subject, see the April/June 2000 issue of Mind &
Language, featuring "On Quantifier Domain Restriction" by Jason Stanley & Zoltán Gendler Szabó, with commentaries by Kent Bach and Stephen Neale. Some of the same ground is covered in a recent
paper by Scott
Soames, as discussed by Brit Bogaard on her blog Lemmings.